In the core of a nuclear reactor the active elements are constituted by vertically juxtaposed "combustible assemblies" each assembly itself combining a great number of combustible rods and an assembly, movable as a unit for emplacement in the reactor or for its extraction. These are the rods which comprise the combustible member itself.
For a better utilization of the combustible material and to render uniform the distribution of power in the reactor, the assemblies are distributed in three zones with different ranges of enrichment along the zones. One then proceeds with the recharging in successive tiers. In each operation one thus eliminates the elements whose combustible material is spent and which is disposed in the central portion of the reactor. One then replaces it with an equal number of elements from the peripheral portion of the reactor and one supplies this peripheral portion with assemblies containing fresh combustible material. Therefore, each recharging operation comprises a first series of manipulations of the assemblies including lifting, putting in place or transposition at the interior of the reactor.
It is also necessary to note that each combustible assembly is associated with auxiliary members capable of being interposed between the combustible rods and manipulable as a unit for each assembly in the form of a "cluster". The types of these members and the cluster which they form then varies according to the position of the assembly in the reactor. Thus certain assemblies are associated with "control clusters" carrying control rods of absorbent material adapted to regulate the reactivity of the reactor and to assure it's halt in the case of emergency. Other control clusters called "partial clusters" only include absorbent material over a portion of their length. Other clusters called "consumable poison" are introduced into the assemblies not provided with "control clusters" to permit reduction of the concentration of the dissolved boron in the primary water. Certain of these "consumable poison" clusters are also provided with rods containing neutron sources for effecting the starting of the reactor. Finally, certain combustible assemblies receive sealing clusters simply adapted to close the guide tubes which are not occupied by other types of rods.
It is therefore seen that for each recharging operation, it is necessary not only to displace or transpose the combustible assemblies but also most often to change the "cluster" with which each assembly is associated.
In the current conditions of use, the operations of recharging a combustible element require at least two or three manipulation devices. The first is a rolling bridge which is displaced on rails above the plane of the water of the pool of the reactor. It is provided with a telescoping arm which effects extraction from the core of the reactor of a complete assembly with its cluster and transfer of the assembly always under water, and always with its cluster up to the equipment zones disposed at the extremity of the pool of the reactor and comprising a machine for transposing the control clusters and a machine for transposing the sealing clusters. These manipulation devices are utilized to separate the clusters from the assemblies. The spent assemblies are then evacuated without their cluster towards the used combustible pool while the assemblies in transposition are generally equipped with new clusters introduced by the telescopic mast on the bridge, and the assemblies are reintroduced at their new location in the core of the reactor. Similarly, the new combustible assemblies receive their control cluster or sealing cluster at the corresponding equipment zone before being taken by the bridge and introduced into the reactor.
It is seen that each definitive lifting operation for replacement or each transposition leads to two displacements of the bridge between the reactor and the equipment zone which considerably lengthens the time of operations for renewing the combustible material and requires personnel to drive the bridge and the auxiliary apparatus for handling the clusters at the equipment zones.